Advances in DNA testing are allowing people to uncover information about their genetic ancestry and find out where some of their ancestors came from. As an African American, I don’t know where my African ancestors originated from. The only geographic location I can point to as my ancestral home is Tennessee. So I’m fascinated by the potential knowledge I could gain from this new generation of tests for genetic ancestry.
But before I fork over more than $200 for such a test, the skeptic in me needs some answers. What can a DNA test really tell me about where I come from? How do these tests work? And can they be wrong?
Companies that offer genetic testing services for finding out about ancestry use several different testing methods. Lineage-based approaches analyze DNA on the Y chromosome, which is passed down almost unchanged from fathers to sons, or else analyze mitochondrial DNA, which is passed down nearly unchanged from mothers to their children. Small genetic changes in the Y chromosome occur as this information is passed from successive fathers to sons. These changes, if they persist, become markers of descent. Likewise, as mitochondrial DNA is passed down, slight mutations occur, and if these mutations persist, they also become genetic markers that can help distinguish one matrilineal line from another.
Taking these tests is straightforward. A person swipes the inside of his or her cheek for a saliva sample, which is sent to a lab. There, the DNA is extracted, amplified, and analyzed. It is then compared to and matched with DNA samples from a reference database of haplotypes – a set of closely linked genes or DNA polymorphisms – that have been identified in specific populations. If a person’s DNA sequences match certain sequences in the database, the information can be used to determine the populations with which that person shares maternal or paternal ancestry.
“Lineage testing can trace your ancestry back to real existing people who carried that particular DNA type throughout prehistory until today,” explains Peter Forster, a geneticist at the University of Cambridge in the U.K. and cofounder of Roots for Real, a company in Cambridge that uses mitochondrial DNA tests to determine maternal ancestry.
But these methods have a drawback. They account for only a small portion of a person’s ancestry. Mitochondrial testing traces a person’s mother, maternal grandmother, maternal great grandmother, and so on. Similarly, Y chromosome testing traces only one line of a person’s male ancestry, starting with a man’s father, his paternal grandfather, paternal great grandfather, and so forth.
Another strategy for ancestry tracking is admixture testing. This kind of test focuses on the 22 pairs of nonsex chromosomes in every cell. Since one of the chromosomes has been inherited from the person’s mother and one from the father, they contain recombined segments of DNA from all of a person’s ancestors. The test compares an individual’s DNA with specific sequences of DNA that are more prevalent in people from one area of the world than from another area. Admixture testing can determine which of the major bio-geographical population groups a person belongs to – sub-Saharan African, European, East Asian, or Native American. The test results are given as a percentage breakdown.